Despite the widely recognized health impacts of excess body fat in our population, the determinants of individual differences in obesity are not well understood. Studies in mice have led to many breakthroughs in understanding human genetics, including leptin and its receptor, sweet taste receptors, and members of the growth hormone pathway. One source of body size variation we have recently identified is a locus or cluster of loci on mouse chromosome 2 near 160 Mb. We have verified in several crosses that genetic variants in this region relate to large differences in fat mass in mice, and these results are (1) consistent in their direction and magnitude of allelic effect and (2) resilient to changes n environment. To prepare to identify these loci, we used marker-assisted selection to breed two reciprocal chromosome 2 substitution strains that can be used as parents for backcross studies to fine-map the relevant loci (Aim 1). These strains are also a ready-made start point to construct congenics to isolate this region into successively smaller intervals (Aim 2). Once the interval is very small, knockout mice will be measured for fat weight to determine which gene(s) is responsible for these traits (Aim 3). The strategy of Aim 3 makes use of the increased number of knockout mice available as a part of the Knockout Mouse Project (KOMP). The payoff of this project will be to identify a source of natural variation that regulates fat weight in animals. Because many medical problems are due directly or indirectly to changes in body composition, understanding their molecular pathways may point to new avenues to improve human health.